Ken,

It depends on how long you want the repair to last.  The only long term
fix is welding in new metal to replace all the rusted out areas.  When
you get to the pinhole stage the metal is very thin, and usually rusting
from the inside too.

If it is not worth this much effort, wire brush and paint with a rust
killing paint like Corroless from Eastwood, and fill over the Corroless
with polyester filler.  I like catalyzed glazing compound for easy
sanding and finishing in this type of repair.

Applying the Corroless to the metal and putting filler on top will
prevent moisture from ever getting to the weak metal from above, or
contact metal through the filled pinholes from the inside.  In addition,
deep rust is almost always impossible to completely remove, some is
usually left between layers of corroded metal. This will keep on rusting
under filler and paint; the Corroless is supposed to stop this process.
Filler sticks just fine to paint and primers that are in good condition.

(On my "strip to bare metal" paint jobs I first level out large dents,
welds, etc, and apply epoxy primer to the entire car. Then I apply the
fillers on top of the primer.  This process requires that you re-spray
primer after sanding to cover the newly exposed metal, but ensures that
no rusting will occur through a paint scratch over filler, which absorbs
moisture and will conduct it to the bare metal below.  This is more
expensive and labor intensive, so pro refinishers don't use this
method.)

If you can get Corroless on the inside, so much the better.

I have used this technique for several years now with no rust through
problems so far!

Have fun!
--
Stuart MacMillan
Manager, Case Program
800-909-8244 ext 208

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